1. Field of the Invention
The present invention relates to a shading correcting method and a shading correcting apparatus for reducing the irregularity in the density of a read image in an image reading apparatus such as a facsimile or an image scanner.
2. Description of the Related Art
In an image reading apparatus such as a facsimile or an image scanner, so-called shading correction is made so as to compensate for the irregularity in the density between pixels due to, for example, the nonuniformity in illumination of a light source for reading. Specifically, digital data obtained by converting an output analog signal of the image sensor using an analog-to-digital converter (hereinafter referred to as "A/D converter") is subjected to shading correction expressed by the following equation (i): ##EQU2## where K is a constant.
Black reference data is digital data obtained by converting a signal outputted from the image sensor when a deep black reference image is read, and white reference data is digital data obtained by converting a signal outputted from the image sensor when a pure white reference image is read. For example, the black reference data is obtained by turning the light source for reading of the image sensor off to cause the image sensor to perform a reading operation, and the white reference data is obtained by turning the light source for reading on to cause the image sensor to read a white reference plate.
The basic construction for making shading correction is shown in FIG. 39. More specifically, the difference between white reference data WST and black reference data BST (WST-BST) is operated by a subtracter 151. In addition, the difference between input data ID and the black reference data BST (ID-BST) is operated by a subtracter 152. Outputs of the subtracters 151 and 152 are applied to a divider circuit 153, and output data of the subtracter 152 (ID-BST) is divided by output data of the subtracter 151 (WST-BST), to obtain correction data.
In the above described shading correction, however, the maximum value of the input data is the white reference data. When the white reference data is smaller than the maximum output of the A/D converter, therefore, the dynamic range of the correction data is narrowed. For example, if the white reference data is "0001111" even if the A/D converter has data which is seven bits in depth, the correction data can be only substantially changed in the range of four bits. Therefore, the resolution of the density is decreased, so that the density can be represented only at a small number of gray levels.